In recent years, ecological factors and government regulations have played major roles in coatings technology development. One of the main environmental concerns is how to reduce or, even better, totally eliminate the volatile organic compounds (VOC) currently used in coating compositions. Waterborne and powder coatings have received increased attention in recent coatings technology development due to such concerns. Water dispersible urethane, acrylic, alkyds, and epoxy resins are examples of such coatings. However, most of these coating systems are less resistant to chemical and solvent attack, they are lower in gloss and, in many cases, they are substantially more expensive than solvent based systems.
Polyurethanes are particularly important in the coatings industry by virtue of their outstanding properties. One and two-component waterborne urethane systems have been developed for various coatings applications, To reduce the cost, thermoplastic urethane-acrylic hybrid dispersions have received considerable attention. The hybrids possess advantages in terms of performance and cost balance when compared to either the pure urethane dispersion and pure acrylic dispersion. However, the chemical and solvent resistance of the resulting coatings based on urethane-acrylic hybrid dispersions suffer in comparison to other systems.
Representative patent literature which show water dispersible polyurethanes and hybrid systems are as follows:
U.S. Pat. No. 5,173,526 discloses a process for producing an aqueous polyurethane-vinyl polymer (hybrid) coating composition by (a) forming a carboxy-containing, water-dispersible, isocyanate-terminated polyurethane prepolymer, (b) adding a vinyl monomer mixture to the carboxy-containing, water-dispersible, isocyanate-terminated polyurethane prepolymer to make a prepolymer/monomer mixture, (c) adding tertiary amine to the prepolymer/monomer mixture, (d) dispersing the prepolymer/monomer mixture in water, (e) adding an oil-soluble free radical initiator and a chain extender to the aqueous dispersion and (f) completing the chain extension and polymerizing the vinyl monomers by heating the aqueous dispersion. A variety of vinyl monomers, which include lower alkyl (C.sub.1-6) esters of acrylic or methacrylic acid, are suggested as being suited for the hybrid.
U.S. Pat. No. 5,331,039 discloses a coating composition comprising a water dispersible binder composition containing two different water-dispersible polyol polymers and a polyisocyanate component dispersed therein. One of the water dispersible polymers is a polyester or polyester resin containing urethane, carboxyl and/or hydroxyl groups, and the other is a hydroxylated polyacrylate or a hydroxyacrylate-grafted polyester that can be crosslinked by a hydrophilic polyisocyanate emulsified in water.
U.S. Pat. No. 5,075,370 discloses a two-component, aqueous coating composition comprising a polymer polyol component containing carboxylic or sulfonic acid groups and a polyisocyanate component. An example of the polymer polyol is a hydroxyacrylate polymer. The polyisocyanate is emulsified therein for the purpose of effecting crosslinking.
(AU-A-41135/89, discloses two-component coating compositions comprising an aqueous dispersion having at least one hydroxyl containing polymer and a polyisocyanate component. Hydroxyl containing polymers are formed by polymerizing unsaturated monomers containing hydroxy groups, often with monomers having acid groups. The resulting acid groups in the polymer then are neutralized with an amine. A variety of hydroxyacrylates and acrylic esters are suggested as candidates. Aliphatic and aromatic isocyanates, as well as uretdiones and isocyanurates, were suggested as suitable isocyanates for effecting crosslinking. Prior to application the isocyanate aqueous hydroxy polymers are emulsified in the dispersion.
Canadian Application 2,092,157 discloses a water thinnable two component coating composition comprising a polyisocyanate, a polyurethane prepolymer having free NCO groups reacted with low molecular weight or monofunctional alcohols or compounds having reactive hydrogen atoms. Water dispersible polyurethanes incorporating neutralized carboxyl functionality are utilized.
European Patent Application (EP 0562 282 A1 discloses a two component waterbased varnish based upon a blend of (A) an aqueous dispersion of an acrylic polymer containing from 2-4% hydroxy groups and an aliphatic polyurethane resin and (B) a liquid aliphatic polyisocyanate having low volatility, generally incorporating a solvent. The acrylate/polyurethane ratio is 60/40 and the acrylate content is from 15 to 18% of the dispersion.
U.S. Pat. No. 4,927,876 discloses aqueous polymer dispersions of a water dispersible polyurethane and a vinyl polymer, the polyurethane containing diphenylmethane diisocyanate. The water dispersible polyurethane component is prepared by forming an isocyanate terminated prepolymer having carboxyl functionality incorporated therein, chain extending with an amine. The vinyl polymer may be blended with the water dispersible polyurethane or vinyl monomers may be polymerized in the presence of the aqueous polyurethane dispersion. A variety of monomers suited for polymerization are suggested which include acrylates and hydroxyacrylates.